Apache HTTP Server 2.x Thread Safety Issues

When using any of the threaded mpms in the Apache HTTP Server 2.x it is important
that every function called from Apache be thread safe. When linking in 3rd
party extensions it can be difficult to determine whether the resulting
server will be thread safe. Casual testing generally won't tell you this
either as thread safety problems can lead to subtle race conditions that
may only show up in certain conditions under heavy load.

When writing your module or when trying to determine if a module or
3rd party library is thread safe there are some common things to keep in
mind.

First, you need to recognize that in a threaded model each individual
thread has its own program counter, stack and registers. Local variables
live on the stack, so those are fine. You need to watch out for any
static or global variables. This doesn't mean that you are absolutely not
allowed to use static or global variables. There are times when you
actually want something to affect all threads, but generally you need to
avoid using them if you want your code to be thread safe.

In the case where you have a global variable that needs to be global and
accessed by all threads, be very careful when you update it. If, for
example, it is an incrementing counter, you need to atomically increment
it to avoid race conditions with other threads. You do this using a mutex
(mutual exclusion). Lock the mutex, read the current value, increment it
and write it back and then unlock the mutex. Any other thread that wants
to modify the value has to first check the mutex and block until it is
cleared.

If you are using APR, have a look
at the apr_atomic_* functions and the
apr_thread_mutex_* functions.

This is a common global variable that holds the error number of the
last error that occurred. If one thread calls a low-level function that
sets errno and then another thread checks it, we are bleeding error
numbers from one thread into another. To solve this, make sure your
module or library defines _REENTRANT or is compiled with
-D_REENTRANT. This will make errno a per-thread variable
and should hopefully be transparent to the code. It does this by doing
something like this:

#define errno (*(__errno_location()))

which means that accessing errno will call
__errno_location() which is provided by the libc. Setting
_REENTRANT also forces redefinition of some other functions
to their *_r equivalents and sometimes changes
the common getc/putc macros into safer function
calls. Check your libc documentation for specifics. Instead of, or in
addition to _REENTRANT the symbols that may affect this are
_POSIX_C_SOURCE, _THREAD_SAFE,
_SVID_SOURCE, and _BSD_SOURCE.

Not only do things have to be thread safe, but they also have to be
reentrant. strtok() is an obvious one. You call it the first
time with your delimiter which it then remembers and on each subsequent
call it returns the next token. Obviously if multiple threads are
calling it you will have a problem. Most systems have a reentrant version
of of the function called strtok_r() where you pass in an
extra argument which contains an allocated char * which the
function will use instead of its own static storage for maintaining
the tokenizing state. If you are using APR you can use apr_strtok().

crypt() is another function that tends to not be reentrant,
so if you run across calls to that function in a library, watch out. On
some systems it is reentrant though, so it is not always a problem. If
your system has crypt_r() chances are you should be using
that, or if possible simply avoid the whole mess by using md5 instead.

The following is a list of common libraries that are used by 3rd party
Apache modules. You can check to see if your module is using a potentially
unsafe library by using tools such as ldd(1) and
nm(1). For PHP, for example,
try this:

c-client uses strtok() and
gethostbyname() which are not thread-safe on most C
library implementations. c-client's static data is meant to be shared
across threads. If strtok() and
gethostbyname() are thread-safe on your OS, c-client
may be thread-safe.

Notice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.